Centrifugal pump

ABSTRACT

A centrifugal pump for accommodating entrained gas and vapor at low absolute suction pressures includes an impeller with a central inlet chamber. The impeller has a peripheral wall which is concentric with the impeller&#39;s axis; and, at least one passage extends outwardly from an inlet port in the peripheral wall to an exit port radially spaced from the chamber. Preferably, the pump includes a stationary inlet port device located within the central inlet chamber for separating the spinning impeller from incoming fluid.

TECHNICAL FIELD

The present invention relates to pumps for pumping liquids and mixturesof liquids and gases over a wide range of pressures and flow rates. Inparticular, the invention provides pumps which are capable ofaccommodating entrained gas and vapour at low absolute suctionpressures.

The invention was developed for use with domestic vacuum distillationsystems (desalinators) but it will be appreciated that the invention isnot limited to this particular application.

DEFICIENCIES OF BACKGROUND ART

Problems are encountered when attempting to pump liquids, andparticularly mixtures of liquids and gases, at low absolute suctionpressures and low flow rates using a centrifugal device. These problemsarise largely from the gas and vapour content of the flow, which is moreprominent at lower pressures. These may be dissolved gases whichseparate at low pressures or the gaseous phase of the liquid itself.

At high flow rates in the order of hundreds of litres per minute, theflow velocity is such that entrained vapour tends to be purged from thesystem in the high velocity fluid flow. However, known centifugal pumpsare unable to operate at low pressures of the order of 23 to 25 inchesof mercury and low flow rates of the order of 300 mls per minute. Infact these known pumps are unable to accommodate significant quantitiesof gas and vapour at these pressures, regardless of the flow rate.

Problems are also encountered in providing reliable long lived seals forthese pumps.

Disclosure of the Invention

According to the invention there is provided a centrifugal pump havingan impellor mounted for rotation about an axis, said impellor having acentral inlet chamber with a peripheral wall concentric with said axisand at least one passage extending outwardly from an inlet port in saidwall to an exit port radially spaced from said chamber; wherein,stationery inlet port means are located within said chamber wherein,stationary inlet port means are located within said chamber forseparating the spinning impellor from incoming fluid entering said inletport means thereby to prevent induced rotation of said incoming fluid,said stationary inlet port means including an annular portion concentricwith said axis and in substantial alignment with said impellor inletport, at least one channel extending radially outwardly from theinterior to the exterior of said annular portion for supplying saidincoming of said impellor passage.

In this way the impellor is prevented from imparting a spinning velocityto the incoming fluid which would otherwise tend to separate the liquidfrom any entrained vapour and ultimately choke-off the supply entirely.

In a second aspect of the invention the impellor is located within itsown delivery volume and wholly immersed in previously delivered fluid soas to deliver directly from said impellor to the previously deliveredfluid. The impellor may be arranged at any desired orientation to thevertical. This provides a very simple pump with no peripheral structuresince the delivery is directly from the impellor to the surroundingfluid.

The invention may also advantageously provide means for slowing theperipheral flow velocity of fluid immediately adjacent the impellorperiphery.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a cut-away and elevation of a pump adapted for pumping waterand entrained air at low pressures and flow rates.

FIG. 2 is a section taken on line 2--2 of FIG. 1.

FIG. 3 is a fragmentary section taken on line 3--3 of FIG. 1.

FIG. 4 is a sectional side elevation of a second pump.

FIG. 5 is a view taken generally on line 5--5 of FIG. 4 with the coverplate removed.

FIG. 6 is a plan view of the pump of FIG. 4, and

FIG. 7 is an enlarged section taken on line 7--7 of FIG. 5.

FIG. 8 is a view similar to FIG. 7 but illustrating an alternativearrangement.

FIG. 9 is a view taken on line 9--9 of FIG. 8.

FIG. 10 is a partly sectioned side elevation of a horizontal axis pumpembodying the invention and incorporating a stationary inlet port means,termed a "shear tube".

FIG. 11 is a view similar to FIG. 10 but illustrating a secondembodiment vertical axis pump incorporating a "shear tube" similar toFIG. 10.

FIG. 12 is a view taken on line 12--12 of FIG. 11.

PREFERRED EMBODIMENTS

Referring firstly to FIGS. 1 to 3, the pump 1 is driven by an electricdrive motor 2 via a horizontally extending central shaft 3. The pumpincludes four major components, a support housing 4, a dividing wall 5,a impellor 6 and a cover plate 7. The impellor 6 is mounted to the motordrive shaft 3 to rotate within a impellor chamber 8 defined by the spacebetween the dividing wall 5 and the end plate 7.

Water and entrained air enter the pump at inlet port 9, passing throughradial passage 10 to an axial passage 11 communicating with the centreof the impellor 6. The impellor 6 is located closely adjacent theadjoining side walls of the impellor chamber 8 and includes a centralbore or chamber 12 communicating with a plurality of radial passages 13extending from the central bore 12 to exit ports 14 spaced around theouter periphery 15 of the impellor 6.

As the impellor rotates, centrifugal action moves the fluid from thepump inlet port 9, outwardly through the impellor passages 13 and intothe annular space 16 surrounding the impellor. From the annular space 16the fluid flows through a port 17 in the dividing wall 5 into a staticchamber 18, finally leaving the pump through the rearwardly directedexit port 19.

Rotation of the impellor in the direction shown also imparts a similarlydirected peripheral flow velocity to the fluid in the annular space 16.This flow velocity produces a centrifugal separation of fluid and vapoursuch that any entrained air tends to cling to the impellor periphery 15.This build-up of entrained air adjacent the impellor peripheryinterferes with flow from the radial passages 13 while tending toaccumulate and remain in the impellor chamber as a fresh supply of waterand entrained air enters to replace the water leaving the impellorchamber through port 17.

The peripheral flow velocity also has the effect of reducing therelative velocity of fluid moving past the radial passage exit ports 14.It is desirable to have this relative velocity in order to augment thecentrifugally induced pressure drop by superimposing a bernoulli effectat the exit port, thereby dropping the pressure still further.

In order to slow the circulating peripheral flow velocity and also topromote the physical removal of segregated air bubbles from closelyadjacent the impellor periphery, the invention provides a scoop 20 withits sharp leading edge 21 located as close as possible to the impellorperiphery. It will be appreciated that the scoop fulfils two functionsin physically removing entrained air and also providing an obstructionin the annular space 16 for slowing the circulating peripheral flowvelocity and thereby improving the pump suction characteristics byincreasing the bernoulli effect at the impellor exit ports 14.

To further improve the pump performance by promoting removal ofentrained air from the impellor chamber, the invention provides arecirculation of substantially air-less water into the peripheral streamupstream of the scoop 20. This is achieved by means of a passage 24through the dividing wall 5 interconnecting the static chamber 18 withthe impellor chamber 8 at a point below the port 17 through which airand water enter the static chamber. Since the flow within the staticchamber is relatively slow, entrained air bubbles are able to separateout from the water in the lower part of the static chamber such that therecirculated flow is substantially depleted of air. The velocity of therecirculating water is preferably kept as slow as possible to preventrecirculation of entrained air along with the water. The addition of thesubstantially airless water into the annular space 16 causes a greaterproportion of entrained air to be removed by the scoop 20 than wouldotherwise have been the case.

FIGS. 4 to 7 illustrate a second pump. Corresponding reference numeralshave been used to identify corresponding integers throughout the variousembodiments.

Pump 40 is similar in many respects to pump 1 except in that the scoop20 is replaced by a diffusor ring 41 which surrounds the impellorperiphery 15 and is spaced closely thereto.

The ring 41 has a plurality of generally radially extending passagesthrough it comprising a first circumferential array of passages 42 whichare centrally located so that they may come into register with the exitports 14 of the impellor 6, and a second circumferential array ofpassages 43 which are arranged in pairs. Each pair of passages 43 isdisposed between adjacent passages 42 with the individual passages 43being axially spaced apart one on either side of the array of passages42 as shown in FIG. 7. The passages 42 enable water to flow directlyfrom the radial passages 13 to the impellor chamber 8, whilst thepassages 43 enable bubbles to escape from the gap between the impellorperiphery 15 and the ring 41 to the impeller chamber 8.

As can be seen in FIG. 5 the passages 42 and 43 are located only in thenine o'clock to eleven o'clock and the one o'clock to three o'clocksectors. The passages 42 and 43 in the nine o'clock to eleven o'clocksector extend radially whilst those in the one o'clock to three o'clocksector are inclined upwardly. It has been found that this configurationprevents or at least reduces undesirable circulatory flow in theimpellor chamber 8. The configuration also causes the flow to begenerally in the direction of an upwardly directed exit port 44, whichreplaces exit port 19 of the first embodiment.

The pump 40 further includes a recirculation passage 45 which extendsfrom the impellor chamber 8 to the radially innermost area of theimpellor 6. Water at a higher pressure in the impellor chamber 8 is ableto flow through the passage 45 to the innermost area of the impellorface which is at a lower pressure, thereby to reduce the tendency ofunwanted air to enter the space between the impellor face and the pumphousing from inlet port 9.

Whilst the static chamber 18 of pump 1 is not necessary, it is used toconduct water which flows through the axial passages 46 to a seal 47 toaffect its lubrication. It will be appreCiated that the static chamber18 could be replaced with a suitable duct.

A third pump is illustrated in FIGS. 8 and 9. This version is similar inmost respects to the pump of FIGS. 4 to 7 and corresponding featureshave been given corresponding reference numerals. However, in this pumpthe passages 42 and 43 in the diffusor ring 41 have been replaced with aplurality of slots 50. The slots may be all radially extending or someor all of them may be inclined in the same way as the passages shown inFIG. 5.

The ring is preferably formed integral with the dividing wall 5 but itmay be separately formed. In this latter case the slots can be cut fromboth sides of the ring in alternating castellated formation.

The impellor of the third pump includes two axially staggered arrays ofequally spaced radial passages 13. For example, a total of 20 radialpassages 13 may be equally spaced around the impellor 6. The slots 50extend a sufficient distance across the diffusor ring 41 to encompassthe passages.

A pump embodying the invention is shown in FIG. 10. In this embodimentthe impellor drive shaft axis 51 is horizontal and the impellor islocated wholly within a delivery tank volume 52. The pump has noperiphery structure since the exit ports 14 deliver directly to thetank.

This embodiment is suitable for low pressure flows at much higher flowrates in the order of many litres per minute. In order to accommodatethe entrained vapour the central chamber 12 of the impellor 6 isprovided with a stationary inlet port means in the form of a "sheartube" 53. The shear tube 53 is stationary and includes a plurality ofdelivery ports 54 arranged around the upper half of the tube.

If the impellor of FIG. 10 is disposed with its axis in a verticalorientation the shear tube ports 54 would be disposed around the centrecircumference of the shear tube. These may be holes as shown in FIG. 10or slots as illustrated in FIG. 11. These stationary ports 54 conveyincoming fluid from the axial inlet passage to the central chamber 12 ofthe impellor 6. This chamber 12 is enlarged slightly as shown by theshallow V-sectioned circumferential groove 55 to facilitate supply offluid to the passages 13.

The shear tube separates the incoming fluid from the spinning impellorand thereby prevents induced rotation of the incoming fluid. This avoids"pre-whirl" - the formation of a gas or vapour pocket along the axialcentreline 51 of the pump inlet due to centifugal motion of the incomingfluid and vapour mix. The shearing effect on the liquid/vapour mix asthis passeS through the shear tube ports 54 and comes into contact withthe spinning inner periphery of the impellor central chamber 12 keepsthe vapour interspersed with the liquid as it enters and passes up theimpellor passages 13.

FIG. 11 illustrates a second embodiment pump similar to the pump of FIG.10 but with the impellor axis vertical. This pump can also operatewholly within the delivery tank 52 without any peripheral structuresurrounding the impellor. In this case the shear tube 53 takes the formof a blind ended extension from a vertically extending axial passage 11.The ports 54 are formed by circumferentially spaced axially elongateslots extending radially outward as best shown in FIG. 12. The sheartube 53 is spaced slightly from the impellor 6 which is itself radiallyslotted to define a plurality of radially extending passages 13. Thisembodiment is particularly suited to high flow rates.

If required, the impellor and shear tube arrangements of FIGS. 10, 11and 12 may be incorporated into the previously described pumps.

Although the invention has been described with reference to specificexamples, it will be appreciated by those skilled in the art that theinvention may be embodied in many other forms.

I claim:
 1. A centrifugal pump having an impellor mounted for rotationabout an axis, said impellor having a central inlet chamber with aperipheral wall concentric with said axis and at least one passageextending outwardly from an inlet port in said wall to an exit portradially spaced from said chamber; wherein, stationary inlet port meansare located within said chamber for separating the spinning impellorfrom incoming fluid entering said inlet port means thereby to preventinduced rotation of said incoming fluid, said stationary inlet portmeans including an annular portion concentric with said axis and insubstantial alignment with said impellor inlet port, at least onechannel extending radially outwardly from the interior to the exteriorof said annular portion for supplying said incoming fluid to saidimpellor passage.
 2. A centrifugal pump according to claim 1 whereinsaid at least one passage is straight.
 3. A centrifugal pump accordingto claim 1 wherein said at least one passage extends radially outward.4. A centrifugal pump according to claim 1 wherein said impellorincludes a plurality of circumferentially spaced outwardly extendingpassages, and said stationary inlet port means include a plurality ofcircumferentially spaced outwardly extending channels.
 5. A centrifugalpump according to claim 1 wherein said stationary inlet port meanscomprises a blind ended extension from an axially extending incomingpassage.
 6. A centrifugal pump according to claim 1 including means forslowing the peripheral flow velocity of fluid immediately adjacent theimpellor periphery.
 7. A centrifugal pump according to claim 6 whereinsaid means for slowing the peripheral flow velocity comprises a scoophaving a sharp leading edge located closely adjacent the impellorperiphery and directed against the direction of impellor rotation.
 8. Acentrifugal pump according to claim 6 wherein said means for slowing theperipheral flow velocity comprises a diffusor ring surrounding theimpellor periphery and spaced closely thereto, said ring including aplurality of outwardly extending passages.
 9. A centrifugal pumpaccording to claim 8 wherein some of said outwardly extending passagesare directed in tangentially opposite directions thereby to reducecirculatory flow.
 10. A centrifugal pump according to claim 7 includingmeans for recirculating substantially vapour-less fluid into theperipheral stream upstream of said scoop.
 11. A centrifugal pumpaccording to any of claims 1 to 9 wherein said impellor is locatedwithin its own delivery volume and wholly immersed in previouslydelivered fluid so as to deliver directly from said impellor to thepreviously delivered fluid.